Method and Apparatus for Image Sensor Packaging

ABSTRACT

Methods and apparatus for packaging a backside illuminated (BSI) image sensor or a BSI sensor device with an application specific integrated circuit (ASIC) are disclosed. A bond pad array may be formed in a bond pad area of a BSI sensor where the bond pad array comprises a plurality of bond pads electrically interconnected, wherein each bond pad of the bond pad array is of a small size which can reduce the dishing effect of a big bond pad. The plurality of bond pads of a bond pad array may be interconnected at the same layer of the pad or at a different metal layer. The BSI sensor may be bonded to an ASIC in a face-to-face fashion where the bond pad arrays are aligned and bonded together.

BACKGROUND

Complementary metal-oxide semiconductor (CMOS) image sensors are gainingin popularity over traditional charged-coupled devices (CCDs). A CMOSimage sensor typically comprises an array of picture elements (pixels),which utilizes light-sensitive CMOS circuitry to convert photons intoelectrons. The light-sensitive CMOS circuitry typically comprises aphoto diode formed in a silicon substrate. As the photo diode is exposedto light, an electrical charge is induced in the photo diode. Each pixelmay generate electrons proportional to the amount of light that falls onthe pixel when light is incident on the pixel from a subject scene. Theelectrons are converted into a voltage signal in the pixel and furthertransformed into a digital signal which will be processed by anapplication specific integrated circuit (ASIC).

A CMOS image sensor, or simply a CMOS sensor, may have a front sidewhere a plurality of dielectric layers and interconnect layers arelocated connecting the photo diode in the substrate to peripheralcircuitry, and a backside having the substrate. A CMOS sensor is afront-side illuminated (FSI) image sensor if the light is from the frontside of the sensor, otherwise it is a back-side illuminated (BSI) sensorwith light incident on the backside. For a BSI sensor, light can hit thephoto diode through a direct path without obstructions from thedielectric layers and interconnect layers located at the front side.This helps to increase the number of photons converted into electrons,and makes the CMOS sensor more sensitive to the light source.

Three-dimensional (3D) integrated circuits (ICs) may be used to achievea high density required for current applications, such as image sensorapplications. When a CMOS sensor is packaged in a 3D IC, a CMOS sensorand its related ASIC may be bonded to a carrier wafer in parallel, whichmay take a larger area for the carrier wafer. Therefore there is a needfor methods and systems to reduce the package area for CMOS sensorsbonded to related ASICs.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1( a) and 1(b) illustrate a view of the top side of a CMOS sensor,and an ASIC, with a plurality of bond pad arrays;

FIGS. 2( a)-2(b) illustrate methods and apparatus for connecting bondpads within a bond pad array of a CMOS sensor;

FIGS. 3( a)-3(d) illustrate methods and apparatus for the configurationsof bond pads within a bond pad array of a CMOS sensor;

FIGS. 4( a)-4(b) illustrate top views for bond pads within a bond padarray of a CMOS sensor;

FIG. 5 illustrates a cross-section view of the bonding of a CMOS sensorand an ASIC; and

FIGS. 6( a)-6(c) illustrate a cross-section view of the bonding of abond pad array of a CMOS sensor and a bond pad array of an ASIC.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the embodimentsof the disclosure, and do not limit the scope of the disclosure.

The present disclosure discloses methods and apparatus for packaging abackside illuminated (BSI) image sensor or a BSI sensor device with anapplication specific integrated circuit (ASIC). According to anembodiment, a bond pad array may be formed in a BSI sensor where thebond pad array comprises a plurality of bond pads electricallyinterconnected, wherein each bond pad of the bond pad array is of asmall size which can reduce the dishing effect of a big bond pad. Theplurality of bond pads of a bond pad array may be interconnected at thesame layer of the pad or at a different metal layer. The BSI sensor maybe bonded to an ASIC in a face-to-face fashion where the bond pad arraysare aligned and bonded together.

FIG. 1( a) illustrates a surface view of the top side of a CMOS sensor10, or a sensor 10. The sensor 10 may be made up of millions ofcomponents such as active devices and passive devices. The sensor maycomprise an active area 11 and a bond pad area 12 where a plurality ofbond pad arrays such as 13 and 14 are located. Some other forms of bondpads may exist in the bond pad area 12 as well. The active region 11,containing the majority of the high density, active circuitry of theCMOS sensor 10, may be located in a relative center area of the sensor10.

A plurality of bond pad arrays such as 13 and 14 are located along theperiphery of the CMOS sensor 10. A bond pad array 13 or 14 may comprisea plurality of bond pads. As shown in FIG. 1( a), the bond pad array 13comprises a total of nine bond pads arranged as a 3*3 matrix format,including a first bond pad 13_1, and a second bond pad 13_2, which areseparated from one another. The bond pad 13_1 and 13_2 are electricallyconnected together with each other and with other remaining bond padswithin the bond pad array 13. The interconnection between the bond padswithin a bond array 13 may be made in various ways, such as in a metallayer below the top surface. The bond pad array 13 is further separatedfrom another bond pad array 14 by a larger distance compared to thedistance between bond pads within the same bond array. The bond padswithin the bond pad array 14 are connected electrically together, butare electrically isolated from the bond pads within the bond pad array13. The number of bond pad arrays 13 and the number of bond pads withina bond pad array are for illustrative purposes only and are notlimiting. There may be other numbers of bond pad arrays, and each bondpad array may comprise a different number of bond pads.

Similarly, a view of the top side of an application specific integratedcircuit (ASIC) 110 is shown in FIG. 1( b). The ASIC 110 may be relatedto the sensor 10 to process digital or analog signals generated by thesensor 10. The ASIC 110 may be made up of millions of components such asactive devices and passive devices. These components are initiallyisolated from each other, formed on an underlying silicon substrate, andare later interconnected together by metal interconnect lines to formthe functional circuit. Typical interconnect structures include lateralinterconnections, such as metal lines or wirings, and verticalinterconnections, such as vias and contacts. The side of the siliconsubstrate on which the integrated circuit is formed may be referred toas the top side or the front side of the ASIC 110. The ASIC 110 may bereferred to as an integrated circuit device, a circuit, a device, acircuit device, a die, or in any other terms known to those skilled inthe art.

As illustrated in FIG. 1( b), bond pad arrays 113 and 114 are located inthe bond pad area 112 along the periphery of the ASIC 110. In the centerof the integrated circuit device 110 is the active region 111 containingthe majority of the high density, active circuitry of the ASIC 110. TheASIC 110 may further be surrounded by a guard ring region, placedoutside of bond pad arrays 113 that protects the ASIC 110, not shown.

The bond pad arrays 13 in FIGS. 1( a) and 113 in FIG. 1( b) may functionto supply data, retrieve data, test a device, and supply variousvoltages for testing or programming. An exemplary bond pad arraycomprises a test mode bond pad array not connected to and separate froman operational mode bond pad array. When the test sequence is complete,the test mode of the test mode bond pad array is disabled. The two bondpad arrays are then connected during wire bonding, and the resultingjoined bond pad arrays function in an operational mode.

The sensor 10 and the ASIC 110 may be bonded together by a method suchas adhesive bonding, direct copper bonding, or direct oxide bonding. Ifthe direct copper bonding is used, the sensor 10 and the ASIC 110 arebonded by applying a high pressure, so that the bond pad arrays 13 ofthe sensor 10 and the bond pad arrays 113 of the ASIC 110 which may becopper pads, are bonded together. By directly bonding the sensor IC 10and the ASIC 110 together, a conventional carrier wafer is not needed,therefore decreasing die area and cost.

Traditionally, some large bond pads may be used in bonding the sensor 10and the ASIC 110. For example, one large bond pad may be used instead ofthe bond pad array 13. These large bond pads suffer from problems. Inthe formation of large bond pads, chemical mechanical polish (CMP)processes typically involved. As a result, during the CMP processes forforming large bond pads, a dishing effect occurs, which causes thecenter regions of bond pads to be polished more than the edge regions.The dishing effect may adversely affect the reliability of the bonding.With the dishing effect, only small portions of bond pads are bonded toeach other, and hence the bonding is less reliable. Moreover, thecurrent that may reliably flow through the bonded area is reduced due tothe reduced bond area. A bond pad array 13 comprising a plurality ofsmaller bond pads as shown in FIG. 1( a) can reduce or eliminate suchproblems.

FIG. 2( a) illustrates a CMOS sensor 10, which is a portion of a wafer.The CMOS sensor 10 includes a semiconductor substrate 20 with a frontside and a backside. The substrate 20 may be formed of commonly knownsemiconductor materials such as silicon, silicon germanium, or the like.The sensor 10 may comprise a grid or array of pixel regions or sensorelements formed on the substrate 20. One such a sensor element or apixel region 60 is shown in FIG. 2( a) as an example.

The pixel region 60 may comprise a photosensitive diode, sometimesreferred to as a photo diode, which may generate a signal related to theintensity or brightness of light that impinges on the photosensitivediode. The photosensitive diode may be a pinned layer photo diodecomprising a p-n-p junction. A non-pinned layer photo diode mayalternatively be used. Any suitable photo diode may be utilized with theembodiments, and all of these photo diodes are intended to be includedwithin the scope of the embodiments. The pixel region 60 may furthercomprise a transistor, which may be a transfer transistor, a resettransistor, a source follower transistor, or a select transistor. Thetransistor may comprise a gate dielectric adjacent the substrate, a gateelectrode over the gate dielectric, and spacers along the sidewalls ofthe gate dielectric and gate electrode.

The substrate 20 may further comprise a plurality of isolation areas,not shown, to separate and isolate various devices formed on thesubstrate, and also to separate the pixel regions from other logicregions of an image sensor.

On the backside of the substrate 20, a micro-lens and a color filterelement 70, may be formed on a dielectric layer over the backside of thesubstrate 20 for color imaging applications. The micro-lens lenses maybe located between the color filter and the backside of the substrate,such that the backside-illuminated light can be focused on thelight-sensing regions. The micro-lens converges light illuminated fromthe backside of the substrate to the photo diode. Associated with eachof the color filter elements is a corresponding micro-lens. The colorfilter elements and associated micro-lenses may be aligned with thephotosensitive elements of the sensor layer using alignment marks.

On the front side of the substrate 20, more integrated circuits such astransistors and other devices such as capacitors, resistors, and thelike, not shown, may be formed. An interconnect structure 24 may beformed over the devices. The interconnect structure 24 may include aplurality of dielectric layers 26, including, but not limited to,inter-metal dielectrics (IMD), passivation layers, and the like. TheIMDs 26 may be formed of low-k dielectric materials with k values lessthan, for example, about 2.5. Metal lines 30 and vias 32 are formed inthe plurality of dielectric layers 26, and may be formed of copper usingthe well-known damascene process, or formed of other metals such asaluminum, tungsten, silver, or the like.

Interconnect structure 24 interconnects the underlying integratedcircuits, and connects the integrated circuits to respective bond pad 42and bond pad array 52, which are formed on the front side of thesubstrate 20. The bond pad 42 may be formed in the active area 11 or inthe bond pad area 12 and bond pay array 52 may be any of the bond padarrays 13 or 14, or others shown in the bond pad area 12 in FIG. 1( a).The bond pad 42 may have a smaller size in width W1 compared to theoverall size W2 of the area covered by the bond pad array 52. Forexample, the W2 may be in a size about several-ten um˜100 um. Throughoutthe entire sensor 10, and possibly the entire wafer, a threshold lateraldimension is predetermined, and any bond pad with a lateral dimension(either width and/or length) greater than the threshold lateraldimension will have a bond pad array form as shown in the bond pad array52, while any bond pad having lateral dimensions (width and/or length)less than the threshold lateral dimension will be one solid bond pad asthe bond pad 42. For example, a predetermined dimension of a size 20 ummay serve as a threshold that any pad has a size of 20 um or over, whichcould be a width or a length of the pad, may be formed as a bond padarray, otherwise it can be formed as a single pad.

There are only 3 bond pads shown in the bond pad array 52. There may beother number of bond pads within the bond pad array 52. Each bond padwithin the bond pad array 52 is connected electrically to each other sothat the bond pad array 52 functions as one contact, sometimes, it maybe referred to as a patterned bond pad 52. A bond pad within the bondpad array 52 may be of a similar size, in a range about 5-20 um widthand length. A first bond pad within the bond pay array 52 may beseparated from its neighboring bond pad by a distance in a range about5-20 um.

The electrical connections between bond pads within the bond pad array52 are made through metal line 30′ and vias 32′. Metal line 30′ and vias32′ may be in the metallization layer (or inside the passivation layer)immediately underlying bond pad 52, which may be a top metallizationlayer (commonly referred to Mtop) or a redistribution layer.Alternatively, the electrical interconnection between the bond padswithin the bond pad array 52 may be provided in any of the underlyingmetallization layers ranging from the bottom metallization layer(commonly referred to as M1) to the top metallization layer Mtop.

Through vias (TV) 40 and 50 extend through substrate 20, andinterconnect the features on the front side to the backside of substrate20. Through vias 40 and 50 may be, for example through silicon vias,through oxide vias, or the like. Generally, as noted above, thesubstrate 20 may comprise a variety of materials, such as a siliconsubstrate, an oxidized substrate, or the like. Through vias 40 and 50represent vias extending through the substrate 20. TV 40 is electricallyconnected to bond pad 42, and TV 50 is electrically connected to bondpad array 52. Accordingly, bond pad 42 and TV 40 may be used to carry arelatively small current, for example, a signal current, while bond padarray 52 and TV 50 may be used to carry a relatively great current, forexample, a power supply current. Bond pad 42 may be electricallydisconnected from bond pad array 52.

FIG. 2( b) illustrates an alternative embodiment where the bond padswithin the bond pad array 52 are connected together at the backside ofthe substrate 20. On the backside of sensor 10, bond pads 62 and 72 maybe formed. Instead of using one TV to interconnect features on oppositesides of substrate 20, more than one TV 50 ₁, 50 ₂, and 50 ₃ mayelectrically interconnect the bond pad array 52 to a bond pad 72 at thebackside of the substrate. In an embodiment, bond pads 62 and 72 havethe specification similar to that of bond pad 42 and bond pad array 52,respectively. Accordingly, bond pad 72 may be larger than bond pad 62.

FIGS. 3( a)-3(d) are top views of several possible designs of bond padarray 52 or 72 as shown in FIG. 2( a) and FIG. 2( b). Both FIGS. 3( a)and 3(b) show bond pad array 52 including a bond pad 52 ₁ separated byopenings 53, and interconnection 52 ₂ for connecting to the bond pad 52₁. Interconnection 52 ₂ are also in the same layer as the bond pad 52 ₁.

On the other hand, FIG. 3( c) illustrates another embodiment in whichthe bond pad array 52 includes a plurality of discrete bond pads 52 ₁separated by openings 53. In the layer in which the bond pad array 52 islocated, there is no electrical connection between discrete bond pad 52₁. Electrical connections are provided through the underlying vias andmetal lines, just as metal line 30′ and vias 32′ shown in FIG. 2( a).With the structure shown in FIGS. 3( a)-3(c), the bond pad array 52 actsas an integrated bond pad, meaning that the connection to any of thebond pad 52 ₁ is equivalent to the connection to other bond pads withinthe bond pad array.

Combination schemes may be formed to include both the embodiments shownin FIGS. 3( a)-3(b), and the embodiment shown in FIG. 3( c). In thecombination schemes, some of the bond pads 52 ₁ are interconnectedthrough interconnection 52 ₂ as groups, while different groups of theinterconnected 52 ₁ are disconnected from each other. An exemplaryembodiment is shown in FIG. 3( d), in which each column of bond pads 52₁ are interconnected, while the columns are discrete. Again, there willbe at least one underlying via 32′ connected to each of the columns, andthe vias 32′ are interconnected, as shown in FIG. 2( a).

Openings 53, as shown in FIGS. 3( a) through 3(d), may at leastpartially be filled with a dielectric material. Referring to FIG. 2( a),at least lower portions of openings 53 are filled with the material ofthe top one of the plurality of dielectric layers 26.

In the case the embodiments as shown in FIG. 3( a)-3(b) are adopted,there may be more than one via 32′ connected to the bond pads 52 ₁, asshown in FIG. 2( a). Alternatively, since bond pads 52 ₁ are alreadyinterconnected, there may be only one via 32′ shown in FIG. 2( a) formedand connected to only one of the bond pads 52 ₁, as shown in FIGS. 3(a)-3(b). However, in the case the embodiments as shown in FIGS. 3(c)-3(d) are adopted, each of the discrete bond pad 52 ₁ has to have anunderlying via 32′ connected to it. Otherwise, the ones without theconnecting vias 32′ will not be able to connect with other bond pads.

Depending on where the cross-sectional view as shown in FIG. 2( a) isobtained, the cross-sectional view of bond pad array 52 may appear assolid bonds as shown in FIG. 4( a), or appear as one continuous pad asindicated by FIG. 4( b), wherein FIG. 4( a) may be the cross-sectionalview taken along a plane crossing line 4A-4A in FIG. 3( a), while FIG.4( b) may be the cross-sectional view taken along a plane crossing line4B-4B in FIG. 3( b).

FIG. 5 illustrates a face-to-face bonding of a sensor 10 and an ASICchip 110. The ASIC chip 110 may have a same or a different structure asthat shown in FIGS. 2 through 4. Advantageously, by adopting theembodiments of the present disclosure, bond pad arrays 52 and 152 may beformed without the concern that the dishing effect may occur, and hencethe resulting bonding is more reliable, and can conduct greatercurrents. The bond pad array 152 is formed on a bond pad area of theASIC 110, at the front side of the substrate of the ASIC. The bond padarray 152 comprises three bond pads as illustrated in FIG. 5, separatedby openings between each other. There may be other number of bond padswithin the bond pad array 152. The bond pad of the bond pad array 152 isdirectly bonded and connected to the bond pad of the bond pad array 52,in a one-to-one fashion as shown in FIG. 5. While this may be thepreferred fashion for bonding the sensor 10 and the ASIC 110 together,other forms of bonding may be possible. For example, the bond pad array152 may have a different configuration from the bond pad configurationof the bond pad array 52. The bonding of the bond pad array 52 and thebond pad array 152 may be formed on one of the bond pad for each bondpad array, such as a first bond pad of the bond pad array 52 and a thirdbond pad of the bond pad array 152 are bonded together, or in addition,a second bond pad of the bond pad array 52 and a fourth bond pad of thebond pad array 152 are bonded together.

FIGS. 6( a)-6(c) illustrate three possible cross-sectional views of thebonding between the bond pad arrays 52 and 152, wherein the differentviews are the results of taking cross-sectional views at differentpositions, such as the one shown in FIGS. 4( a) and 4(b), and/or theresults of forming bond pad arrays 52 and 152 with the same or differentstructures.

In the disclosed structure of the sensor 10, the illuminated lightduring applications may not be limited to a visual light beam, but canbe extended to other optical light such as infrared (IR) and ultraviolet(UV), and other proper radiation beams.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

1. A backside illuminated (BSI) sensor device, comprising: a substratewith a front side and a backside; a photosensitive diode within anactive area of the substrate; and a first bond pad array at the frontside of the substrate, wherein the first bond pad array comprises afirst bond pad and a second bond pad separated by an opening andelectrically connected to each other, and every electrical componentconnected to the first bond pad is also electrically connected to thesecond bond pad, and wherein the first bond pad and the second bond padare configured to directly connect to an external connector.
 2. The BSIsensor device of claim 1, wherein the first bond pad array furthercomprises additional bond pads which are all electrically connected toeach other and to the first bond pad and the second bond pad.
 3. The BSIsensor device of claim 1, wherein a dielectric material fills into atleast a portion of the opening separating the first bond pad and thesecond bond pad.
 4. The BSI sensor device of claim 1, wherein the firstbond pad and the second bond pad are of a similar size.
 5. The BSIsensor device of claim 1, further comprising a micro-lens and a colorfilter on the backside of the substrate.
 6. The BSI sensor device ofclaim 1, wherein the first bond pad and the second bond pad of the firstbond array are electrically interconnected in a same layer in which areformed the first bond pad and the second bond pad.
 7. The BSI sensordevice of claim 1, wherein first bond pad and the second bond pad arediscrete, an interconnection in a different layer than the first bondpad and the second bond pad electrically interconnects the first bondpad and the second bond pad.
 8. The BSI sensor device of claim 7,further comprising an additional bond pad of the first bond pad array,wherein the first bond pad is electrically interconnected to theadditional bond pad in a same layer in which are formed the first bondpad and the additional bond pad.
 9. The BSI sensor device of claim 1,further comprising a through via connected to the first bond pad array.10. The BSI sensor device of claim 9, further comprising an additionalbond pad array on the backside of the substrate, wherein the additionalbond pad array is electrically connected to the through via.
 11. The BSIsensor device of claim 1, further comprising a solid bond pad in a samelevel as, and electrically disconnected from, the first bond pad array,wherein the solid bond pad is smaller in width than the first bond padarray.
 12. The BSI sensor device of claim 1, further comprising: an ASICchip comprising an ASIC substrate, and a second bond pad array at afront side of the ASIC substrate, wherein the second bond pad arraycomprises a third bond pad and a fourth bond pad separated from butelectrically connected to each other, and functioning as one electriccontact; and wherein the first bond pad of the first bond pad array andthe third bond pad of the second bond pad array are directly bondedtogether.
 13. The BSI sensor device of claim 12, wherein: the secondbond pad of the first bond pad array and the fourth bond pad of thesecond bond pad array are bonded together.
 14. An integrated circuit(IC) package, comprising: a sensor device comprising a substrate with afront side and a backside; a photosensitive diode within an active areaof the substrate; and a first bond pad array at the front side of thesubstrate, wherein the first bond pad array comprises a first bond padand a second bond pad separated by an opening and electrically connectedto each other, and every electrical component connected to the firstbond pad is also electrically connected to the second bond pad; an ASICchip comprising a substrate, and a second bond pad array at a front sideof the substrate, wherein the second bond pad array comprises a thirdbond pad and a fourth bond pad separated by an opening and electricallyconnected to each other, and functioning as one electric contact; thefirst bond pad of the first bond pad array and the third bond pad of thesecond bond pad array are directly bonded together.
 15. The IC packageof claim 14, wherein: the second bond pad of the first bond pad arrayand the fourth bond pad of the second bond pad array are directly bondedtogether.
 16. The IC package of claim 14, wherein first bond pad and thesecond bond pad are of a similar size.
 17. The IC package of claim 14,wherein the sensor device further comprises a micro-lens and a colorfilter on the backside of the substrate.
 18. The IC package of claim 14,wherein first bond pad and the second bond pad are electricallyinterconnected in a same layer in which are formed the first bond padand the second bond pad.
 19. The IC package of claim 14, wherein firstbond pad and the second bond pad are discrete, formed in a first layerand are electrically connected by an element formed in a differentlayer.
 20. The IC package of claim 14, further comprising a through viaconnected to the first bond pad array.